The present invention relates to a listening device for a hands-free system.
The present application claims priority from Japanese Patent Application No. 2002-337411, the disclosure of which is incorporated herein by reference.
Hands-free kits for mobile communication terminals such as mobile phones and PHSs are generally known. FIG. 1 shows one example of an in-vehicle hands-free system: When the communication terminal (a) receives a call, its adaptor (b) automatically connects the call after one to three rings. The user speaks with the caller through a microphone (c). After the caller has hung up, the adaptor automatically cuts the call.
With this system, the voice from the adaptor's built-in speaker (d) can be heard throughout the vehicle. Some other hands-free kits use a single earphone instead of the speaker (d), which may, however, be problematic for in-vehicle applications, because the driver cannot hear ambient sounds well with one ear plugged. To resolve this problem, there has been developed a listening device using a bone conduction actuator which transmits voice signals through a human bone. For example, Japanese Patent Application Laid-Open No. 2001-56693 shows a listening device having an actuator built in a seat headrest. Japanese Patent Application Laid-Open No. 2002-191469 shows a listening device actuator built in a seat back or a seat cushion.
These conventional listening devices using a bone conduction actuator (hereinafter, simply called as “actuator”) have the following problems:
The actuator needs to be pressed against a surface of a human body with constant and appropriate pressure so as to achieve favorable signal transmission through the bone. However, because the actuator is fixed in part of the seat in the above prior art, the pressure changes depending on the posture of the user on the seat or head movements, the actuator outputs cannot always be transmitted to the human bone in an optimal manner.
Also, part of the human body against which the actuator is to be pressed has various curved shapes depending on the users. Because the actuator is fixed in the above listening devices, it cannot always be brought in tight contact with the surface of the human body in various shapes. There occurs thus a difference in the level of signal transmission because the actuator is not contacted to the user's body part in a constant manner.
Moreover, it is very likely that the user moves his/her body or head frequently while driving or during the use of the listening device. Put differently, the user will feel uncomfortable if he/she cannot move his/her body or head freely during use. In this respect, the prior art actuators are uncomfortable to use because they are fixed and the user must fix his/her body or head so as to hear the caller's voice well.